[ovs-dev] [PATCH v9 10/15] ovs-thread: Do not quiesce in ovs_mutex_cond_wait().

[Daniele Di Proietto]

ovs_mutex_cond_wait() is used in many functions in dpif-netdev to
synchronize with pmd threads, but we can't guarantee that the callers do
not hold RCU references, so it's better to avoid quiescing.

In system_stats_thread_func() the code relied on ovs_mutex_cond_wait()
to introduce a quiescent state, so explicit calls to
ovsrcu_quiesce_start() and ovsrcu_quiesce_end() are added there.

Signed-off-by: Daniele Di Proietto <diproiettod at vmware.com>
---
 lib/ovs-rcu.h           | 3 +--
 lib/ovs-thread.c        | 2 --
 vswitchd/system-stats.c | 6 ++++++
 3 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/lib/ovs-rcu.h b/lib/ovs-rcu.h
index 600be0b..8750ead 100644
--- a/lib/ovs-rcu.h
+++ b/lib/ovs-rcu.h
@@ -42,8 +42,7 @@
  * A "quiescent state" is a time at which a thread holds no pointers to memory
  * that is managed by RCU; that is, when the thread is known not to reference
  * memory that might be an old version of some object freed via RCU.  For
- * example, poll_block() includes a quiescent state, as does
- * ovs_mutex_cond_wait().
+ * example, poll_block() includes a quiescent state.
  *
  * The following functions manage the recognition of quiescent states:
  *
diff --git a/lib/ovs-thread.c b/lib/ovs-thread.c
index 3c065cf..26dd928 100644
--- a/lib/ovs-thread.c
+++ b/lib/ovs-thread.c
@@ -253,9 +253,7 @@ ovs_mutex_cond_wait(pthread_cond_t *cond, const struct ovs_mutex *mutex_)
     struct ovs_mutex *mutex = CONST_CAST(struct ovs_mutex *, mutex_);
     int error;
 
-    ovsrcu_quiesce_start();
     error = pthread_cond_wait(cond, &mutex->lock);
-    ovsrcu_quiesce_end();
 
     if (OVS_UNLIKELY(error)) {
         ovs_abort(error, "pthread_cond_wait failed");
diff --git a/vswitchd/system-stats.c b/vswitchd/system-stats.c
index df4971e..129f0cf 100644
--- a/vswitchd/system-stats.c
+++ b/vswitchd/system-stats.c
@@ -37,6 +37,7 @@
 #include "json.h"
 #include "latch.h"
 #include "openvswitch/ofpbuf.h"
+#include "ovs-rcu.h"
 #include "ovs-thread.h"
 #include "poll-loop.h"
 #include "shash.h"
@@ -615,7 +616,12 @@ system_stats_thread_func(void *arg OVS_UNUSED)
 
         ovs_mutex_lock(&mutex);
         while (!enabled) {
+            /* The thread is sleeping, potentially for a long time, and it's
+             * not holding RCU protected references, so it makes sense to
+             * quiesce */
+            ovsrcu_quiesce_start();
             ovs_mutex_cond_wait(&cond, &mutex);
+            ovsrcu_quiesce_end();
         }
         ovs_mutex_unlock(&mutex);
 
-- 
2.1.4